A multitude of different rate-adaptive pacemaker designs have been known and used clinically for years, for which the stimulation frequency or stimulation rate is adjusted in dependence on signals recorded inside the patient's body, which reflect the patient's physiological requirement with respect to the cardiac activity. An overview of the goals pursued in the development of rate-adaptation in pacemaker technology and the paths taken is provided in the relatively early standard work by K. Stangl et al.: Frequency Adaptive Pacemakers, Darmstadt, 1990.
The so-called QT pacemaker is also discussed in this work (pages 255-272), for which the stimulation rate is adjusted with the aid of the time interval between a stimulation pulse and the appearance of the T-spike of the ventricular cardiac action signal that is initiated (evoked) by the stimulus, the so-called QT interval. This interval can be used for the rate control of a pacemaker because it depends on the cardiac frequency as well as the stress and because the dependence on the cardiac frequency is nearly linear, at least for a specific time range. The disadvantages in this case include the detection and analysis of the T-component of the intracardiac ECG, which is rather problematic in practical operation owing to the low signal amplitude and not very distinctive signal shape and, above all, the danger of positive feedback based on the fact that an increase in the stress as well as the frequency leads to a shortening of the QT interval.
According to EP 0232 528 B1 and the EP 0237 767 B1, the stimulation rate is controlled by a comparison between an integrated cardiac action potential and a predetermined target value or comparison value. On closer examination, the procedure amounts to an analysis of the time interval, starting with the stimulus, to the end of the R-spike of the ventricular ECG ("depolarization gradient duration"). However, in practical operation this signal duration cannot be determined with any reliability for a signal encumbered with interference and, especially, with zero line fluctuations.
Thus, it is the object of the invention to provide a rate-adaptive pacemaker of the aforementioned generic type with improved usefulness in practical operations.